inline assistant: Mention ability to add context with @ in the placeholder (#41950)

This has been possible in the inline assistant for ages now and maybe
you didn't know because we didn't say anything about it! This PR fixes
that by including that you can @-mention context on it the same you can
in the agent panel.

Release Notes:

- N/A

crates/agent_ui/src/inline_prompt_editor.rs

@@ -260,10 +260,10 @@ impl<T: 'static> PromptEditor<T> {
 
         let agent_panel_keybinding =
             ui::text_for_action(&zed_actions::assistant::ToggleFocus, window, cx)
-                .map(|keybinding| format!("{keybinding} to chat ― "))
+                .map(|keybinding| format!("{keybinding} to chat"))
                 .unwrap_or_default();
 
-        format!("{action}… ({agent_panel_keybinding}↓↑ for history)")
+        format!("{action}… ({agent_panel_keybinding} ― ↓↑ for history — @ to include context)")
     }
 
     pub fn prompt(&self, cx: &App) -> String {